Discharge lamps are commonly defined and known in the art as all the lamps in which the emission of a radiation, which can be visible or ultraviolet, takes place as a consequence of the electric discharge in a gaseous medium. The discharge is triggered and sustained by the potential difference applied to two electrodes placed at the opposed ends of the lamp.
The cathodes for lamps can have various shapes, for example filaments or spiral wound filaments, or other shapes. A particularly advantageous cathode shape is the hollow cathode. Hollow cathodes have generally the shape of a hollow cylinder that is open at the end facing the discharge zone, and closed at the opposite end. As is well known to those skilled in the art, one advantage given by the hollow cathodes with respect to other cathode shapes is their lower potential difference (of about 5%-10%) required to “light” the discharge Another advantage of the hollow cathode is a lower intensity of the “sputtering” phenomenon by the cathode, namely the emission of atoms or ions from the cathodic material that can deposit on adjacent parts, among which include the glass walls of the lamp, thus reducing the brilliancy of the lamp. Examples of lamps with hollow cathodes are described for instance in patents U.S. Pat. Nos. 4,437,038, 4,461,970, 4,578,618, 4,698,550, 4,833,366 and 4,885,504 as well as in the published Japan patent application 2000-133201, which are hereby incorporated by reference.
It is also known by those skilled in the art that in order to ensure a proper operation of these lamps throughout their lives, it is necessary to ensure the consistency of the mixtures forming the gaseous medium of the discharge. These mixtures are, in general, mainly formed by one or several rare gases, such as argon or neon, and in many cases some milligrams of mercury. The composition of these mixtures can vary from the desired one, because of both the impurities remaining in the lamp from the production process, and those released during time by the same materials forming the lamp or permeating inward from the walls thereof. Impurities present in these mixtures can damage the working of the lamp in various ways. For example, oxygen or oxygenated species can react with mercury to form HgO, thus removing the metal from its function. Hydrogen can cause discharge striking difficulties (and consequently lighting difficulties of the lamp) or change the operating electrical parameters of the lamp, increasing its energy consumption.
In order to eliminate these impurities it is known by those skilled in the art to introduce a getter material into the lamps. Getter materials have the function of fixing the impurities through a chemical reaction, thus removing them from the gaseous medium. Getter materials widely used for this purpose are the zirconium-aluminum alloys described in U.S. Pat. No. 3,203,901; the zirconium-iron alloys described in U.S. Pat. No. 4,306,887; the zirconium-vanadium-iron alloys described in U.S. Pat. No. 4,312,669; and the zirconium-cobalt-mischmetal alloy described in U.S. Pat. No. 5,961,750 (mischmetal is a mixture of rare earth metals). All four of these U.S. patents are hereby incorporated by reference. These getter materials are generally introduced in the lamps in the form of getter devices formed by powders of material that are fixed to a support. Usually, getter devices for lamps are formed by a cut-down size of a supporting metal strip, flat or variously folded, onto which the powder is fixed by rolling; an example of such a getter device for lamps is described in U.S. Pat. No. 5,825,127, which is hereby incorporated by reference.
As is generally known by those skilled in the art, in some cases the getter device is formed by a getter material pill simply inserted into the lamp. It is highly preferable when a getter is fixed to some constituting element of the lamp because a getter that is not fixed does not lie generally in the hot areas of the lamp, and so its gas-absorbing efficiency decreases. Further a getter device can interfere with the light emission. The device is accordingly almost always fixed (in general by spot welding), for instance to the cathodic support, whereas in some cases a suitable support is added to the lamp. In all cases, however, additional steps are required in the production process of the lamp. In addition, some lamps have an extremely reduced diameter, such as those used for backlighting the liquid crystal screens, which have diameters not larger than 2-3 millimeters. In a case with such a narrow diameter it is difficult to find a suitable arrangement of the getter device within the lamp, and the assembling operations for the device may become extremely difficult.